This disclosure relates generally to fabrication of optical devices, and in particular but not exclusively, relates to fabrication and placement of spacers in an optical projection device, such as a device having liquid crystal on silicon (LCOS) light modulators.
LCOS devices, such as LCOS light modulators, are an important component of an optical projection system. LCOS devices, typically embodied in chips for use as xe2x80x9cmicro-display screens,xe2x80x9d can eventually substitute for cathode ray tubes (CRTs) for a monitor or for a television.
One of the important parameters for a high quality LCOS device is the uniformity of the cell gap (sometimes referred to as the xe2x80x9ccell spacingxe2x80x9d) in the LCOS device (e.g., the LCOS xe2x80x9ccellxe2x80x9d). In an LCOS device, the cell gap is the space between the upper and lower substrates of the device, with the liquid crystal material being contained within the cell gap. For liquid crystal displays (LCDs), the upper and lower substrates are typically made from a glass material. For LCOS devices, the upper substrate is made from glass, and the lower substrate is made from silicon.
It is common knowledge that a consistency in the thickness and/or uniformity of the liquid crystal material within the cell gap must be achieved in order to obtain a high-quality optical image on the screen. If there is non-uniformity, then colors that should be red may suffer a discernible color shift, for instance, in the display. Spacers are used to help achieve this uniformityxe2x80x94the spacers act as a structural support between the upper and lower substrates to keep the cell gap between the substrates uniform in thickness.
In the manufacturing of LCDs, the practice is to apply spacers having sizes of 3-5 microns in width (either through a wet or dry process) within the cell by spraying the spacers onto the lower substrate, before coupling on the upper substrate with gasket material to form a hermetic seal. Thereafter, liquid crystal is deposited in the cell gaps formed between the spacers and the substrates, via use of vacuum processes.
This spacer manufacturing technique, however, produces unsatisfactory results for LCOS devices. This is because while human eyes cannot gauge actual sizes of the spacers, the spacers sprayed on a small LCOS chip will be enlarged hundreds of times in the projection image. The spacers create xe2x80x9cdead spotsxe2x80x9d in the resulting image. The spacers become so obvious that the resulting image is undesirable, and the LCOS product will not sell well in the marketplace.
Because the visible spacers in the viewing area degrade from the quality of the LCOS device, many commercially available LCOS devices are fabricated without spacers (e.g., are xe2x80x9cspacerlessxe2x80x9d). However, this process is not a guarantee for quality or yieldxe2x80x94because with larger size variations of silicon substrates or with thinner cell gaps, it is mechanically difficult to support constant spacing uniformity when coupling two large substrates. In fact, this can become a serious issue if yield is significantly reduced due to the higher number of LCOS devices that will need to be discarded for having poor uniformity.
To alleviate this problem for LCOS devices, techniques using inherited built-in spacers have been used. Such inherited built-in spacers are formed by etching SiO2 spacers on the silicon substrate itself. However, while the resulting structure does achieve uniformity of the cell gap and thus uniformity of the image quality, the spacers are too glaringly obvious as viewers pay more attention and get closer to the image. In other words, the spacers are relatively large in size and are often formed on each and every pixel in many types of LCOS cells, and as a result, become very visible when the image is magnified. The individual image generated from each cell is peppered with xe2x80x9cdead spotsxe2x80x9d caused by the spacers in that cell, and these dead spots become much more pronounced when a composite/combined image is obtained from individual images from multiple cells, since the dead spots exactly overlap/superimpose over each other in the combined image.
Attempts to address this dilemma include fabrication of cells where the spacers are not formed on every pixelxe2x80x94the overall number of spacers is reduced. However, the spacers that are formed are nevertheless formed in the same pixel location on every cell. Thus, while the overall number of dead spots that xe2x80x9cpepperxe2x80x9d the original image may be reduced, the severity of the dead spots from the existing spacers remains unchanged, since the combined image still exactly superimposes and combines the dead spots from each cell. The overall image quality is therefore still unacceptable.